ERROR: Error installing compass. Failed to build gem native extension.

I ran into this error when running buildout on MacOSX:
Fetching: ffi-1.9.3.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing compass:
    ERROR: Failed to build gem native extension.

    /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
checking for ffi.h... no
checking for ffi.h in /usr/local/include,/usr/include/ffi... yes
checking for ffi_call() in -lffi... yes
checking for ffi_prep_closure()... yes
checking for ffi_raw_call()... no
checking for rb_thread_blocking_region()... yes
checking for rb_thread_call_with_gvl()... yes
checking for rb_thread_call_without_gvl()... yes
checking for ffi_prep_cif_var()... no
creating extconf.h
creating Makefile

make "DESTDIR=" clean

make "DESTDIR="
compiling AbstractMemory.c
compiling ArrayType.c
compiling Buffer.c
compiling Call.c
Call.c:303:5: warning: implicit declaration of function 'rb_thread_call_without_gvl' is invalid in C99 [-Wimplicit-function-declaration]
    rbffi_thread_blocking_region(call_blocking_function, data, (void *) -1, NULL);
    ^
./Thread.h:78:39: note: expanded from macro 'rbffi_thread_blocking_region'
# define rbffi_thread_blocking_region rb_thread_call_without_gvl
                                      ^
1 warning generated.
compiling ClosurePool.c
compiling DataConverter.c
DataConverter.c:43:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
1 warning generated.
compiling DynamicLibrary.c
compiling ffi.c
compiling Function.c
Function.c:479:33: warning: incompatible pointer types passing 'VALUE (void *)' to parameter of type 'void *(*)(void *)' [-Wincompatible-pointer-types]
        rb_thread_call_with_gvl(callback_with_gvl, &cb);
                                ^~~~~~~~~~~~~~~~~
Function.c:102:46: note: passing argument to parameter 'func' here
extern void *rb_thread_call_with_gvl(void *(*func)(void *), void *data1);
                                             ^
Function.c:563:9: warning: implicit declaration of function 'rb_thread_call_without_gvl' is invalid in C99 [-Wimplicit-function-declaration]
        rb_thread_call_without_gvl(async_cb_wait, &w, async_cb_stop, &w);
        ^
Function.c:738:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
3 warnings generated.
compiling FunctionInfo.c
compiling LastError.c
compiling LongDouble.c
compiling MappedType.c
compiling MemoryPointer.c
compiling MethodHandle.c
compiling Platform.c
compiling Pointer.c
compiling Struct.c
compiling StructByReference.c
compiling StructByValue.c
compiling StructLayout.c
compiling Thread.c
compiling Type.c
compiling Types.c
compiling Variadic.c
linking shared-object ffi_c.bundle
clang: error: unknown argument: '-multiply_definedsuppress' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
make: *** [ffi_c.bundle] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/nateaune/Dropbox/code/adhocracy/parts/rubygems/lib/ruby/gems/1.8/gems/ffi-1.9.3 for inspection.
Results logged to /Users/nateaune/Dropbox/code/adhocracy/parts/rubygems/lib/ruby/gems/1.8/extensions/universal-darwin-13/2.0.0/ffi-1.9.3/gem_make.out
rubygems: Command failed with exit code 1: /Users/nateaune/Dropbox/code/adhocracy/parts/rubygems/bin/gem install --no-rdoc --no-ri --bindir=/Users/nateaune/Dropbox/code/adhocracy/parts/rubygems/bin --version 1.0.0.alpha.19 compass -- 
While:
  Installing rubygems.
Error: System error
liqd / adhocracy

ERROR: Error installing compass. Failed to build gem native extension. #861
